Focal segmental glomerular sclerosis (FSGS) is an often progressive form of nephrotic syndrome responsible for 10-15% of end-stage renal disease in children. Patients with FSGS are often resistant to oral steroid therapy, necessitating the use of very aggressive alternative treatments (high-dose intravenous steroids, cyclophosphamide, plasmapheresis). Some patients will respond with remission only after months of this intensified therapy. It would be of great value to be able to predict the severity of disease early in its course, to avoid subjecting patients with less malignant disease to overly aggressive therapy, while starting such therapy promptly in patients likely to have the most resistant disease. Our study is designed to examine the glomerular hemodynamics, macromolecular permeability and quantitative pathology (stereology) of pediatric patients with steroid-resistant nephrotic syndrome or biopsy-proven FSGS, in order to distinguish at disease onset those patients who will require aggressive therapy from those in whom a more extended course of routine oral steroids may eventually lead to persistent remission.